1. Field of the Invention
The present invention is related to a surface light-emitting device using a surface light-emitting element, in particular, an EL (Electro-Luminescence) element.
2. Description of the Related Art
In recent years, in addition to LED (Light Emitting Diode) elements, EL (Electro-Luminescence) elements that are surface light-emitting elements, have become popular as new light-emitters.
Surface light-emitting devices using EL elements are already being commercialized in the forms of a backlight of a mobile phone, an audio player, or the like, or as a small-sized display. In addition, EL displays using EL elements are being studied and commercialized as thin, large-size displays that may potentially replace liquid crystal displays and plasma displays.
In addition, a light emitted by an EL element has a wide emission spectrum and is therefore close to natural light. Consequently, a surface light-emitting device using an EL element is also suitable as a light fixture.
In a surface light-emitting device, an EL element is provided on a transparent substrate. An EL element is configured such that a transparent electrode (anode) with optical transparency, a light-emitting layer, and a metal electrode (cathode) are laminated in order.
Generally, only approximately 20% of light emitted at an EL element is emitted to the outside. The remaining 80% is not emitted to the outside and is not used. A specific breakdown of unused light is as follows. Approximately 20% of the emitted light becomes trapped inside the transparent substrate and 40 to 50% becomes trapped inside the light-emitting layer. The remaining 10 to 20% is absorbed by the metal electrode or the like.
Light becomes trapped in the transparent substrate because a significant difference between the refractive index of the transparent substrate and the refractive index of a medium outside of the surface light-emitting device such as air causes light radiated from the EL element to be reflected instead of being radiated to the outside of the transparent substrate. In consideration thereof, as an example of related art, there is a method of reducing reflection of light and increasing light to be radiated to the outside of a surface light-emitting device by providing a layer having an intermediate refractive index between the refractive index of a transparent substrate and the refractive index of a medium outside of the surface light-emitting device on a surface of the transparent substrate on which an EL element is not provided (Japanese Patent Laid-Open No. 2005-327689).
When a light-emitting device having an LED element is used as a light fixture, the light fixture is particularly well suited as a downlight or a spotlight due to the strong directionality of the LED element. On the other hand, when a surface light-emitting device having an EL element that is a surface light-emitting element is used as a light fixture, the light fixture is particularly well suited to illuminate a certain wide range such as a ceiling light because the directionality of the EL element is not as strong as an LED element. However, while the directionality of the EL element is not as strong as an LED element, the EL element has a near-Lambertian luminous intensity distribution. When a surface light-emitting device having an EL element is used as a light fixture, not too much light is radiated from the side of the light fixture. Therefore, when the light fixture having an EL element is attached to the ceiling of a room and used as a main lighting, the upper part of the room becomes dark. A person feels that a room is bright when the entire room is well lit. Therefore, a light fixture having an EL element and that is used attached to the ceiling of a room has a problem in that the light fixture is unable to evoke a sense of brightness in a person that corresponds to the amount of light actually being radiated to the outside from the light fixture. Accordingly, radiating of light emitted by an EL element to the side of a surface light-emitting device is required. However, while methods disclosed in Japanese Patent Laid-Open No. 2005-327698 and the like are capable of efficiently radiating light emitted at an EL element from a flat surface of a transparent substrate or, in other words, from the front of a surface light-emitting device, the methods have difficulty radiating light to the side of the surface light-emitting device.